1. Field of the Invention
The invention relates generally to semiconductor lithography and pertains, more specifically, to a hydrogen ion microlithography process.
2. Description of the Prior Art
Recently, there has been an extremely rapid growth in the application and fabrication of microelectronics. Microelectronics fabrication generally concerns using conventional semiconductor lithography to produce various micrometer-sized discrete semiconductor devices, integrated circuits and solid-state devices. Generally, in semiconductor lithography, a high resolution integrated circuit pattern is formed in a resist overlaying a semiconductor substrate. A permanent micrometrer-size device structure is formed with the resist pattern acting as a mask by subtractive etching or removal or by additive deposition of metals or insulators.
To continue, the application of microelectronics presently pervades virtually all aspects of commercial, military, business and leisure activities. Moreover, microelectronics has caused profound changes in such diverse fields as computers, calculators, communications, entertainment, sports equipment and process-control systems. Additionally, the size and performance of micrometer-size devices have substantially improved through microminiaturization. Microminiaturization generally describes the reduction in size of solid state devices to submicron level circuits through semiconductor lithography.
For example, the sizes of electronic circuits in a pocket calculator have been reduced from those that fit in a large, desk-top machine to those that fit on the head of a pin. To take another example, a basic functional electrical element of a microelectronic circuit is a transistor. Microminiturization has made it possible to fabricate thousands of transistors in a single chip. This reduction in transistor size has also dramatically increased the operational speed of microelectronic circuits and has lowered the cost per circuit by several orders of magnitude.
Despite this rapid growth, many applications need even higher performance levels, higher functional density, higher reliability and lower production cost. Two promising ways to achieve the desired goals have emerged. The first is by submicrominiaturization, or making the microdevices even smaller. The second is by making the integrated circuit chips as large as possible to stave off forming smaller linesize, submicrometer geometries and device dimensions. Linesize generally concerns the width of features at the resist, while the chip generally refers to the thin-film semiconductor material on which the microcircuits are formed.
However, the need to produce larger chips is significantly limited by the defect density of lithography processing operations. Defect density normally is associated with an average density of fatal defects or contaminates on the chip's surface. For example, a dust particle usually leaves a pinhole with metal in a circuit line used for lift-off, and such a defect can be the cause of a short. The defects can originate from the chip itself, etching, process equipment, storage boxes, personnel, human error or defects in a mask-to-mask pattern. As the chips become larger and the lines become closer, the number and size of the defects must be reduced for good yield.
Since the defect density limits the growth and size of the chip, producing smaller microdevices with closer spaced, narrower features or lines is an increasingly popular way to increase the functional density. The size of the lines usually is a strong function of exposure. Exposure normally involves subjecting the resist to some form of electromagnetic radiation. The radiation causes a differential change in some resist property such that specific line patterns can be formed during resist development. As previously mentioned, development generally involves the removal or additive deposition of a metal or insulator. For these reasons, exposing and developing the resist are dominant areas that provide limitations to reducing the linewidths and the associated device dimensions. Thus, concentration will be hereinafter focused on the needs and problems of the prior art from a resist and processing viewpoint.
To address the needs and problems of resist processing in submicrominiaturization, the microelectronic industry is attempting to refine the dominant semiconductor lithography or photolithography processing procedures. Unfortunately, the desired smaller linewidths, submicrometer device dimensions and geometries usually are beyond the capabilities of photolithography.
For example, a photolithographic mask usually contains an open or transparent pattern. Ultraviolet light is transmitted through the pattern to expose corresponding resist regions for subsequent development. Unfortunately, diffraction effects from the mask openings and reflection effects within the resist frequently degrade the quality of the replicated mask image. The diffraction effects usually occur when the mask is separated from the resist, when high resolution is demanded and when the linewidth is reduced to being comparable to the wavelength of the ultraviolet light. Resolution generally concerns resolving the finest linewidths associated with the wavelength of the ultraviolet light source. Additionally, when the mask is placed relatively close to the resist, irregularities on the resist often cause defects at the mask surface. Such defects frequently result in corresponding defects in the next resist exposed with that mask.
To cope with the problems of diffraction, the microelectronic industry has employed projection photolithography. Projection photolithography normally employs a shorter radiation wavelength to expose the resist and a mask that forms an object in an optical system. The optical system projects an image either in real size or demagnified onto the resist. The shorter radiation wavelength beneficially reduces the diffraction effects. However, spherical aberration effects in lenses of the optical system normally undesirably limit the resolution of this replication process.
In an attempt to alleviate many of the aforesaid problems in photolithography, the microelectronic industry has turned to high-energy (shorter wavelength) radiation exposure systems. For purposes of the present application, high-energy radiation exposure systems are systems with an exposure energy greater than 1000 eV. The conventional sources of such systems are electron beams (e-beams), ion beams and X-rays, which use shorter wavelengths and higher energy photons. The high-energy radiation sources can be employed to either focus a beam of electron energy to a spot or cause the electron energy to be collimated and masked to flood expose the resist.
A number of publications discussing the use of such conventional exposure systems include: an abstract entitled, Focused Ion Beam Scans Small Structure, Test & Measurement World, p. 16 (1988); Darryl W. Peters, Keeping America Competitive, Examining Competitive Submicron Lithography, Semiconductor International, pp. 96-100 (1988); Irwin Goodwin, Compact X-Ray Lithography Machines Generate Hope for Semiconductors, Physics Today, pp. 49-52 (1988); Joseph Grenier, Wafer Fabrication Equipment Five Year Forecast, Solid State Technology, pp. 67-70 (1988); and William Thurber, Photolithography's Heir Still Not Obvious, Says Interface Keynote, Semiconductor International, p. 15 (1988).
In e-beam lithography, which is normally considered the dominant high-energy radiation source, the beam exposes the resist where it strikes and locally changes its characteristics. Subsequent resist development can either selectively remove the exposed resist regions or remove the unexposed regions. However, proximity effects are the primary limitation that prevents obtaining satisfactory linewidths with the e-beam lithography. Proximity effects describe pattern fidelity degradation. This degradation is primarily caused by electron scattering and secondary electron generation in the resist and the chip. For instance, the proximity effects create an exposed volume of resist that is wider than the diameter of the impingent beam.
In X-ray lithography, continuous X-rays are normally produced by electron bombardment of a fixed or rotating anode. Unfortunately, faster organic resists are usually required since the X-ray sources are frequently too weak for present single-film organic resist. For example, generally, no practical single-film organic resist can be exposed rapidly enough to be competitive with projection photolithography.
Likewise, conventional ion lithography, as opposed to the hydrogen ion microlithography of the present invention, suffers problems similar to those of X-ray lithography. These problems are also usually associated with the newer emerging projection ion lithography. For these reasons, the application of conventional high-energy radiation exposure sources such as ion beam lithography is often restricted to fabricating, measuring and repairing photo mask. The publication of T. D. Cambria and N. P. Economou, Mask and Circuit Repair With Focused-Ion Beams, Solid State Technology, pp. 133-136 (1987), explains the use of focused-ion beam technology for mask and circuit repair.
The exposure, development and processing problems relating to organic-based resist have motivated the microelectronic industry to look for a resistless process. A resistless process would directly form circuit components and eliminate all the resist process steps. Generally, the major conventional energy sources existing in resistless, lithography technology are high-energy X-rays, lasers, e-beams and ion beams. However, resistless lithography is comparatively in its infancy. Moreover, the beams of the major high-energy sources normally are difficult to spread over large surface areas.
Several solutions to the problems associated with organic resist exposure, development and processing are disclosed in U.S. Pat. Nos. 4,569,124; 4,601,778; 4,615,904; and 4,377,437. For example, U.S. Pat. No. 4,569,124 discloses forming thin conducting lines in a two-layered inorganic material such as silicon or aluminum overlying a layer of refractory metal with a high-energy, ion-beam implantation source. The two-layered material behaves as a resist. In U.S. Pat. No. 4,601,778, portions of a polysilicon film are initially exposed to either an oxygen plasma or a hydrogen plasma, and thereafter, the entire polysilicon film is exposed to a polysilicon etchant and etched without the need to employ a mask. In U.S. Pat. No. 4,615,904, a patterned film is deposited onto a substrate using a laser energy source or a low-power, focused e-beam source. Lastly, in U.S. Pat. No. 4,377,437, a high-powered ion source for implanting indium, gallium or gold ions is used to define features directly on an inorganic layer of a solid-state precursor device, for example SiO.sub.2 or Si.sub.3 N.sub.4, which serves as a mask.
In a development involving gallium ion implantation, P. H. La Marche and R. Levi-Setti, Amorphous silicon as an inorganic resist, SPIE Vol. 471, Electron-Beam X-Ray and Ion-Beam Technique for Submicrometer Lithographics 111, pp. 60-65 (1984), discloses that negative tone images can be produced in glow-discharge amorphous silicon hydride by selective gallium-ion implantations. Also, the gallium implanted amorphous silicon exhibits a greatly reduced etch rate. Regarding the concept of reducing the resistance to etchants through ion implantation, the publication of P. A. O'Connell, Formation of Resistive Films by Ion Bombardment, Colloquium on Ion Implantation, London England, p. 7 (1970) discusses the formation characteristic of resistance in aluminum films subjected to oxygen ion bombardment.
The present invention, however, represents yet another different development and solution, which employes a low-energy hydrogen ion source, and which results in unexpected improved microlithography process procedures, particularly with respect to eliminating all of the organic resist process steps. The attributes of the present invention are reflected in the following objects.